The present invention relates to ducting for an external topping combustor in a gas turbine power plant. More specifically, the present invention relates to ducting for directing hot gas from an external combustor to a turbine that employs water/steam cooled walls.
The high efficiency, low capital cost and short lead time of gas turbine based systems make them particularly attractive to electric utilities as a means for producing electrical power. However, traditionally, gas turbine operation has been limited to expensive, sometimes scarce, fuels--chiefly distillate oil and natural gas. As a result of the ready availability and low cost of coal, considerable effort has been expended toward developing a gas turbine system for generating electrical power which can utilize coal as its primary fuel. One area in which these efforts have focused concerns systems in which the combustion of coal is carried out in a pressurized fluidized bed combustor, hereinafter PFBC.
In one of the simplest gas turbine/PFBC power plant arrangements, ambient air, compressed in the compressor section of the gas turbine, serves to fluidize the bed and provides combustion air for the PFBC. After combustion in the PFBC, the air, now at a high temperature and vitiated by the products of combustion and entrained particulate matter, is exhausted from the PFBC. The air then flows through a gas cleanup system wherein much of the particulate matter is removed. The air is then directed to the turbine section of the gas turbine where it is expanded, thereby producing useful shaft power. After expansion, the vitiated air exhausted from the turbine is vented to atmosphere.
The thermodynamic efficiency of such a system is poor, however, due to the need to limit the bed temperature, and hence the temperature of the air entering the turbine section, to approximately 870.degree. C. (1600.degree. F.) in order to optimize capture of the sulfur in the coal and avoid carryover of harmful alkali vapors into the turbine. This is in contrast to modern conventional gas or liquid fuel fired gas turbines, which can operate with turbine inlet gas temperatures as high as 1425.degree. C. (2600.degree. F.). As is well known in the art, increasing the temperature of the gas entering the turbine section increases the power output and efficiency of the gas turbine. Hence to achieve maximum efficiency, it has been proposed to employ a separate topping combustor--that is, a combustor external to the gas turbine--to raise the temperature of the air leaving the PFBC to the temperature required for maximum efficiency in the turbine.
Although the system discussed above offers the possibility of efficient use of coal in a gas turbine based system, certain considerations make it extremely difficult to utilize such a separate topping combustor. The primary area of difficulty arises due to the need to transport the very high temperature gas from the topping combustor to the turbine section of the gas turbine. Ducts capable of carrying such hot gas would be subject to high temperature and thermal stress, especially if, as is optimal from the thermodynamic standpoint, the gas is heated in the topping combustor to a temperature suitable for use in a modern high efficiency gas turbine (i.e., approximately 1425.degree. C. (2600.degree. F.)). One approach to solving this problem involves forming the duct from a high temperature alloy. However, such alloys are expensive and difficult to weld. Moreover, their life under the conditions to which they will be subjected in such a duct will likely negatively impact the availability of the power plant.
Accordingly, it would be desirable to provide a durable duct capable of transporting hot gas from an external topping combustor to the turbine section of a gas turbine.